Start by Tube rolling.
Replace the 6N9P used for the SRPP front end with a 6SL7
Bigggest immediate improvement from replacing the 6N8P driver and substituting 6SN7.
Bypass that 22uF Electrolytic in the driver cathode with a 220nF/400V polypropylene capacitor.
Bypass that 4.7uF Polyester cap on the SRPP supply with a 47nF or 100nF polyproylene.
After that its redesign / rebuild time to extract further improvements.
OH almost forgot some of these amps exhibited a hum problem - If yours does then bypass the cathode resistor in the bottom triode in the SRPP.
Check the bias, exactly 1 volt across the 12 Ohm 845 cathode ressitors is 83 mA which is 75 Watts Anode dissipation (The 845 Anode Voltage is 890V on Aussie 250V AC Mains Input). Thats the maximum I'd feel safe running.

Here is the schematic. - looks like a fairly ORDINARY circuit BUt these amps with the above minimal mods actually sound very good indeed. Would probably make a good platform for a front end / driver redesign (say 6SN7 followed by 6V6/EL34 in Triode Mode. or even 6AU6).